1. Field of the Invention
This invention relates to a method for amplifying electric signals, an amplifier circuit and an application of this amplifier circuit as the modulator stage of a subsequent amplifier. This invention particularly relates to the amplification of low frequency modulation signals of a broadcast transmitter.
2. Description of the Prior Art
In an amplifier and in particular in a modulator amplifier for high output power, the direct-current power supplied must be converted into output power with the highest possible efficiency. With very high output powers, push-pull amplifiers are used in so-called class B or class C modes of operation almost without exception. Theoretically, the efficiency of class B operation is 78.5%. With class B low-frequency amplifiers efficiencies of around 70% are achieved in practice. The additional losses are caused by the residual voltage and the quiescent current in the output stages.
If the modulation factor m of an amplitude modulated high-frequency oscillation drops below 1, which is the case where modulation is effected by means of music and speech, the achievable efficiency is much lower still. With low modulation factors down to approximately m=0.4 which are typical with broadcast program modulation, the losses are relatively high with respect to the power output; the efficiency of a modulator with a class B amplifier is then around 27%.
Since the low-frequency power P.sub.mod output of a modulation amplifier is proportional to the square of the modulation factor, P.sub.mod =P.sub.t m.sup.2 /2, where P.sub.T =carrier power, the losses become noticeable with the typical modulation factors and the total efficiency of a transmitter drops rapidly within the range of m=0 to m=0.6.
A class B anode modulator which can be used for large transmitters is known, for example, from Eugen Philippow, Taschenbuch Elektrotechnik, Volume 3, VEB Verlag Technik, Berlin 1978, page 721, in which modulator the modulation voltage is superimposed on the direct anode voltage for the power output stage. The amplitude of the alternating output voltage available at the antenna output is then nearly proportional to the anode voltage. The modulation voltage is provided by a modulation amplifier operated in push-pull class B mode via a modulation transformer. In this arrangement the center tap of the primary winding and one end of the secondary winding of this modulation transformer are at a constant anode potential. One disadvantage of this class B anode modulator consists in high losses occurring with a low modulation factor since the modulation amplifier is always supplied with the full anode voltage.
In German Offenlegungsschrift No. 2,342,714 a grid modulation circuit for radio transmitters is disclosed in which the anode voltage of the output stage of a radio transmitter is altered in synchronism with the change in the average power of a modulation signal. Simultaneously, a control signal is derived from a modulator, which is connected via a driver stage to control this output stage, for a controllable rectifier, at the output of which the altered anode voltage is available.